DESCRIPTION (adapted from the application) Complications of vascular access in patients treated with chronic hemodialysis are a major source of morbidity. Currently, polytetrafluoroethylene (PTFE) grafts are the most prevalent form of vascular access for hemodialysis in the United States. The most frequent complication of these grafts is thrombosis. Consequences include further access procedures, missed dialysis treatments, and temporary vascular access devices. The costs of maintaining vascular access are approximately one billion dollars a year. In a small study, dipyridamole was shown to decrease the incidence of thrombosis in new PTFE grafts. The objective of this proposal is to determine if the use of dipyridamole is efficacious in decreasing the need for intervention and in prolonging PTFE graft survival in patients undergoing chronic hemodialysis. This will be a prospective, randomized, double blinded trial involving five Clinical Centers. The entrance criteria include patients with a functioning (prevalent) or new (incident) PTFE graft with an access flow of >800 ml/min. One hundred twenty five chronic hemodialysis patients from this center will undergo randomization to receive dipyridamole 75 mg TID or placebo TID. If the graft flow (measured monthly) decreases to <800 ml/min and there is >25 percent decrease in flow compared to baseline, patients will undergo diagnostic fistulography. If a significant lesion is found (as defined by strict criteria), the patients will undergo a radiological or surgical intervention. The primary outcome will be the time from initiation of drug until the first intervention for thrombosis or declining access flow rates. The first secondary outcome will be the time from initiation of drug until the graft is irreversibly lost due to thrombosis. The other secondary outcome will be the number of radiological and surgical interventions required to maintain graft patency normalized to patient-year follow-up. If dipyridamole beneficially affects these outcomes, use of the drug in hemodialysis patients with PTFE grafts should decrease patient morbidity and lower healthcare costs.